make options for credo configurable (#2337)

* Add credo --strict option

If a user sets 'let g:ale_elixir_credo_strict=1' it will run credo with
--strict instead of suggest.  The default (0) is to run as suggest.

* Added credo docs

doc/ale-elixir.txt

@@ -72,6 +72,18 @@ g:ale_elixir_elixir_ls_config                   *g:ale_elixir_elixir_ls_config*
     \ }
 <
   Consult the ElixirLS documentation for more information about settings.
+===============================================================================
+credo                                                        *ale-elixir-credo*
+
+Credo (https://github.com/rrrene/credo)
+
+g:ale_elixir_credo_strict                           *g:ale_elixir_credo_strict*
+
+  Type: Integer
+  Default: 0
+
+  Tells credo to run in strict mode or suggest mode.  Set variable to 1 to
+  enable --strict mode.
 
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